opnoense (our version is 17.7.6-amd64, but I guess other versions are also affected) seems to use always "PLAIN" Authentication mechanism.If the E-Mail-Server does not support "PLAIN", E-Mail-Notifications will fail - typically with "Authentication mechanism not supported".Now - guess what ... Exchange does support plaintext-logins when configured correctly, but only using the method "LOGIN" ...

if I change this line to$smtp->authentication_mechanism = "LOGIN";I can send e-mail-notifications via "smtp.office365.com" port 587 and the option "Enable STARTTLS". But I guess this will break Notifications for other mailservers.